The economic concept of cost differs from the everyday notion of a monetary payment. In economics, the cost of doing something is what you have to give up to be able to do it. In this sense, you can face a cost even if you do not have to pay anyone anything. Economists refer to this as an opportunity cost.
Suppose that a farmer has 15 acres of land that can produce 20 bushels of corn per acre or 10 bushels of wheat per acre. She currently is producing 100 bushels of corn and 100 bushels of wheat.
(You might want to go through the calculations to convince yourself that she can indeed produce 100 bushels each of corn and wheat on this amount of land.)
What is the opportunity cost to the farmer, measured in bushels of corn, of producing 1 additional bushel of wheat?